Testing device



Oct. 31, 1933.

A. LUDW IG TESTING DEVICE Filed March 30. 1952 FIG 2 A TTORNEV PatentedOct. 31, 1933 UNITED STATES 1,933,274 'rssrmo DEVICE Arthur Ludwig, NewYork, N. Y., assignor to Bell Telephone laboratories, Incorporated, NewYork, N. Y., a corporation of New York Application March 30, 1932.Serial No. 602,094

2 Claims.

This invention relates to a testing device and more particularly to adirect reading device which is adapted for measuring the percentageclosure of contacting devices used for the transmission 5 of impulsesand the like.

The object of the invention is to provide a testing device which iseflicient and reliable wherein the percentage make period of thecontacting device under test is readily indicated.

The general elements of my invention comprise a source of currentsupply, a. variable resistance, an impedance circuit and a currentmeter. It is based upon the use of an impedance interposed between thesource of current serially connected with the contacts to be tested, andan output receiving circuit comprising the current reading meterflexibly controlled through the aforesaid variable resistance foradjusting the current therethrough to what is required for its fullscale deflection. The electrical constants of the impedance, which isessentially a low-pass filter, are so selected that the currentdelivered to the current meter from the impedance is a constant andaverage value of the current impulses delivered into the impedance bythe impulsing contacts. If the current meter gives a deflection which isproportional to the current flowing through it then it follows that thedeflection will be proportional to the average value of the currentdelivered into the impedance by the impulsing contacts.

be deflected in proportion to the average value of current deliveredinto the impedance and the reading of the -current meter, in divisions,will be equal to the percentage relation which the average currentdelivered to the impedance by the impulsing'contacts'bears to the totalcurrent delivered when the contacts are held steadily closed. As theaverage current bears the same percentage to the total current as themake period of the impulse bears to the total impulse (closed periodopen period) the current meter reading in divisions will be equal toutheper cent .make 01 the impulses.

The mathematical explanation of the above relation may be consideredwith reference to the figures of the drawing in which Fig. 1 is a cir-55 cuit diagram disclosing the arrangement of all the elements of theinvention;

Fig. 2 represents the electrical equivalent, in

conventional form, of the elements shown in Fig.

1 and; 00 Fig. 8 indicates the composite voltage wave The current meteris provided with one hundred scale divisions or any through the entirecircuit when the contacts are p n Considering Fig. 2 in which R1 is theregulating resistance, R: the resistance of the current meter and E theinterrupted source of current 5 supply, it is evident that for smalldifferences between resistances 2 and 4, compared to the batterypotential supplied, the input impedance to the filter is the same as theterminating impedance. Hence there will be no reflected voltages 7 whichtherefore may be disregarded.

. The input voltage wave, controlled by the contacts to be tested, is ofa square shape and is illustrated in Fig. 3. In the consideration ofthis voltage wave =The maximum value of the voltage across the inputside of the filter.

a=Time between maximum and zero value of voltage E. I

1r=Time of wave.

21r=Time of full wave.

This wave may then be represented as a function of time E(t), which maybe extended into a Fourier series:

E(t)=A +:2 ;(a sin npt-l-b,, cos npt in which AoiS the constant term and(Zn and bn the constant terms of the harmonics present.

In this case A0 represents the direct current component of the wave and:2:i(a,,, sin npt-I-b cos npt) the alternating current terms, and sincethe meter, as a readable device, is of no use when the needle isvibrating in response. to the un- 5 filtered alternating currentcomponents of the input wave, only the case when the fundamentalfrequency (n=1) is above the cut-01f point of the filter is to beconsidered. This eliminates the alternating current terms and E (t)o=Ao. 100

Since the part played by the per cent "make of the contacts under testis the only part of interest A0 must be determined by integrating intotwo parts; namely from 0 to a and from a to r thus covering all parts ofthe wave which 105 are not repeated.

f E(t)dt+ f E( t)dt= f A dt+ f .4 11: 0 a 0 a from .which During theinterval 0 to a E(t) will be equal to E and during a to 1r E(t) willequal 0, therefore, 115

ill)- 202/211 is the ratio of the "malre to the whole impulse andEquation (1) means that the voltage across the meter is directlyproportional to maize ratio or the per cent "make. Therefore, thereading of the current meter will be an enact "make reading provided, oicourse, that the full scale coincides with the 100 per cent "maize" byreason of the previous circuit adjust ment and also provided that thespeed of the impulse interruption is sufllciently high to prevent themeter needle from vibrating.

Consider now Fig. l, in which is shown the circuit above analyzedwherein variable condensers 5, d and 7 together with inductances 3 andll form the impedance circuit of the impulse tester, 8 is the currentmeter, 2 and i equalizing resistances, 1 is the resistance forcontrolling the potential across the impedance, 10 a source of current,and 11 the relay having contacts 12 whose percentage make" period infollowing the continuous impulses from interrupter 9 are to be measured.

Before any measurements can be made, meter 8 must be calibrated to afull scale division with respect to source of current 10. This is doneby opening key 13 and establishing a steady state current over thefollowing circuit path: ground, contacts 12, variable resistance 1,resistance 2, inductances 3 and 14, meter 8, conductor 15, to thenegative pole of the battery 10. llhe quantity of current now flowingthrough this circuit will depend, of course, on the direct currentresistance of the circuit and the strength of the battery should be suchthat with all the resistonce 1 cut out of circuit, the meter 8 will readbeyond its full scale calibration. Resistance 1 is then varied until themeter needle reads exactly its full scale indication thereby calibratingthe meter and limiting the maximum quantity of current from battery illto what can be read on mid meter. I

The contacts 12 to be tested are now connected either directly orthrough a relay 11, as shown, to a source of impulses as 9. When key 13is closed, an obvious impulm circuit for relay ill is closed causing therelay a operate and release in following the sequence of impulses frominterrupter 9. As the contacts 12 make and break, the needle of meter 11which ordinarily indicates current now reads a value which, if the fullscale is 100, indicates the percentage malre of the total impulse periodas indicated by the above analysis.

' Condensers 5, 6. and 'l are shown as variable in order to provide fullflexibility'for tuning with specific inductance values of coils 3 andit. When a filter impedance comprising unknown inductances and variablecondensers are arranged as shown, the tuning point for no reflex voltageshould be determined by oscillographic studies or other similar means tomake certain that all possibilities of reflex voltages are eliminated.

What is claimed is:

l. A direct reading device for testing the closed period of contactsadapted to transmit rhythmic impulses, comprising a source of currentthrough said contacts, a measuring instrument, variable means forlimiting the steady state current to the maximum required for full scaledeflection of said instrument, and animpedance electrically interposedbetween said source of current and said instrument for averaging thecurrent delivered by said impulsing contacts in the same proportionalityto the full scale current as the make period of an impulse bears to thetotal period of said impulse.

2. The method of testing the percentage of the make periods of vibratingcontacts to the entire impulse cycles which consists in calibrating ameasuring instrument having one hundred scale divisions over animpedance circuit connected to a source of direct current so that on Isteady current from the source the instrument will indicate anexact'full scale deflection and then intermittently opening and closingthe impedance circuit by the vibrating contacts to be tested, the steadydeflection of the instrument then indicating directly the percentage ofthe make periods to entire cycles of impulses transmitted by thecontacts under test.

ARTHUR LUDWIG.

